1. Field of the Invention
The invention relates to an image pick-up device comprising an imaging lens system, a radiation-sensitive detection system comprising discrete detection elements arranged in a regular pattern with a given period, and a low-pass filter for suppressing image frequencies which exceed the spatial sampling frequency of the detection system.
2. Description of the Related Art
A device of this type may form part of an electronic video camera or of an electronic photocamera and is known, for example, from U.S. Pat. No. 4,575,193.
A sensor termed "CCD" (Charge-Coupled Device) is currently used in such cameras. Such a sensor comprises a, for example, two-dimensional matrix of small radiation-sensitive elements which are integrated with an electronic circuit with which the elements are read line or matrix-sequentially. The radiation-sensitive elements have dimensions of the order of, for example 10.times.10 .mu.m.sup.2 and a sensor may comprise several hundred rows and columns of such elements. Such a sensor performs, as it were, a spatial sampling of the image applied thereto, the spatial sampling frequency being determined by the spatial frequency of the sensor matrix. Due to this image sampling, the image reconstructed from the image sensor signals will exhibit a Moire effect because the image formed on the sensor has spatial frequencies which are higher than the sampling frequency. This Moire effect, also referred to as aliasing, results in a pattern of blocks and flicker of the reconstructed image.
To prevent the Moire effect it has already been proposed, for example, in U.S. Pat. No. 4,575,193, to arrange a low-pass filter between the imaging lens system and the detection system, which filter blocks the image frequencies exceeding half the sampling frequency. In the known device, the low-pass filter comprises one or more birefringet plates, for example, quartz plates which split a beam incident thereon into, for example, two beams. This creates on the image sensor, for example, two images which are slightly offset with respect to each other. The resultant composite image is then slightly extended version of the original image, with the extended version having a smaller resolution.
Practice has proved that the birefringent plates cause a reduction of the modulation transfer function of the image pick-up device, which reduces the pick-up quality.